1. Field of the Invention
The present invention relates to an optical functional element as a basic element in optical information processing and optical transmission. More particularly, the present invention relates to an optical functional element of a loading resistor integrated type in which a light emitting portion, a light receiving portion and a loading resistor in a semiconductor are monolithically integrated with each other.
2. Description of the Related Art
Recently, expectation of parallel processing has been increased for processing a large amount of information at a high speed. However, there is a problem about a transmitting delay caused by wiring in a current electronic circuit so that there is a limit in processing speed. Therefore, a method for transmitting information by using light is devised to overcome the limit in electric wiring.
For example, as is well known, such an optical transmission element is constructed by an element provided by combining a phototransistor with a light emitting diode (see A. Sasaki et al., IEEE Trans. Electron Devices, Vol. 35, No. 6, pp. 780-786, 1988). Another optical transmission element is constructed by a light emitting thyristor having a pnpn structure (see K. Kasahara et al., Appl. Phys. Lett., Vol. 52, No. 9, pp. 679-681). When each of these optical functional elements is independently operated, an electric current flowing through each of the optical functional elements can be controlled by controlling a voltage even when there is no loading resistor. However, when many optical functional elements are arranged in array, it takes cost to attach an individual power source to each of the optical functional elements. Accordingly, such an attaching method is not practically used. Therefore, power is supplied to each of the optical functional elements from a single voltage source. At this time, when there is no loading resistor with respect to each of the optical functional elements arranged in array, an electric current concentratedly flows through only an optical functional element turned on. Accordingly, there is a problem that this optical functional element is broken and another optical functional element is not turned on even when light is irradiated onto this optical functional element. Therefore, a loading resistor is separately required for each of the optical functional elements arranged in array. Merits of a parallel arrangement of the optical functional elements are usefully provided by independently operating the element array. Further, it is desirable to monolithically integrate the optical functional elements with each other so as to make a functional element system compact with reduced cost and improve reliability of this system. Accordingly, it is important to monolithically integrate a loading resistor with each of the optical functional elements arranged in array.
An example of the optical functional element obtained by monolithically integrating the loading resistor therewith is shown by N. Komaba et at., Third Optoelectronics Conference (OEC' 90) Tech. Dig., 12B4-7, pp. 122-123, 1990. In this example, the loading resistor is constructed by a Cr-SiO cermet. Another example of the optical functional element is shown by K. Matsuda et al., IEEE Electron Device Lett. , Vol. 11, No. 10, pp. 442-444, 1990. In this example, the loading resistor is manufactured by using a semiconductor layer below the optical functional element.
It is more desirable to simply manufacture the loading resistor with respect to the optical functional element obtained by monolithically integrating the loading resistor therewith. For example, if a resistance material except for a semiconductor is used for the loading resistor, different processes for manufacturing the loading resistor are required so that the manufacturing processes are complicated. In contrast to this, if the semiconductor is used for the loading resistor, a resistance layer can be simultaneously manufactured when a crystal grows. Accordingly, no additional processes are required and resistivity of a semiconductor layer is easily controlled. However, when there is a loading resistor layer below the optical functional element, it is necessary to remove this optical functional element to the loading resistor layer. In this case, a resistance value of the loading resistor is changed when a thickness of the loading resistor layer is changed. Accordingly, for example, the optical functional element above the loading resistor layer must be removed by using an etching method for precisely controlling a thickness of the optical functional element to the loading resistor layer below the optical functional element. Therefore, it is normally difficult to provide a suitable manufacturing process of the loading resistor layer.